Processes for the treatment of a sour hydrocarbon fraction where the fraction is treated by contacting it with an oxidation catalyst and an alkaline agent in the presence of an oxidizing agent at reaction conditions have become well known and widely practiced in the petroleum refining industry. These processes are typically designed to effect the oxidation of offensive mercaptans contained in a sour hydrocarbon fraction to innocuous disulfides--a process commonly referred to as sweetening. The oxidizing agent is most often air. Gasoline, including natural, straight run and cracked gasolines, is the most frequently treated sour hydrocarbon fraction. Other sour hydrocarbon fractions which can be treated include the normally gaseous petroleum fractions as well as naphtha, kerosene, jet fuel, fuel oil, and the like.
A commonly used continuous process for treating sour hydrocarbon fractions entails contacting the fraction with a metal phthalocyanine catalyst dispersed in an aqueous caustic solution to yield a doctor sweet product. Doctor sweet means a mercaptan content in the product low enough to test "sweet" (as opposed to "sour") by the well known doctor test. The sour fraction and the catalyst containing aqueous caustic solution provide a liquid-liquid system wherein mercaptans are converted to disulfides at the interface of the immiscible solutions in the presence of an oxidizing agent-usually air. Sour hydrocarbon fractions containing more difficult to oxidize mercaptans are more effectively treated in contact with a metal chelate catalyst dispersed on a high surface area adsorptive support-usually a metal phthalocyanine on an activated charcoal. The fraction is treated by contacting it with the supported metal chelate catalyst at oxidation conditions in the presence of a soluble alkaline agent. One such process is described in U.S. Pat. No. 2,988,500. The oxidizing agent is most often air admixed with the fraction to be treated, and the alkaline agent is most often an aqueous caustic solution charged continuously to the process or intermittently as required to maintain the catalyst in the caustic-wetted state.
The prior art shows that alkaline agents are necessary in order to catalytically oxidize mercaptans to disulfides. Thus, U.S. Pat. Nos. 3,108,081 and 4,156,641 disclose the use of alkali hydroxides especially sodium hydroxide for oxidizing mercaptans. Further, U.S. Pat. No. 4,913,802 discloses the use of ammonium hydroxide as the basic agent. The activity of the metal chelate systems can be improved by the use of quaternary ammonium compound as disclosed in U.S. Pat. Nos. 4,290,913 and 4,337,147.
It is also known that materials such as layered double hydroxides (LDH) or metal oxides solid solutions can be used as solid bases on which can be dispersed a metal chelate. These materials are described in U.S. Pat. No. 5,232,887. This patent discloses the use of a solid solution of magnesium oxide and aluminum oxide as well as an LDH identified as hydrotalcite and having the formula EQU Mg.sub.6 Al.sub.2 (OH).sub.16 (CO.sub.3).multidot.4H.sub.2 O
as solid bases. In order to obtain appreciable conversion of mercaptans to disulfides a polar compound such as water or methanol must be added.
In Catalysis Letters, 11, pp. 55-62 (1991), the authors describe the oxidation of 1-decanethiol in water using an LDH in which cobalt phthalocyanine is intercalated between the LDH layers. The process also uses a borate buffer to maintain the pH at 9.25.
In contrast to this art, applicants have discovered that solid solutions of metal oxides can catalyze the oxidation of mercaptans found in hydrocarbon fractions without the use of metal chelates or polar compounds or additional bases. The conditions necessary for oxidizing the mercaptans, i.e., sweetening the hydrocarbon fraction, are the same as those used in conventional sweetening processes. Thus, the instant process has the advantage that it does not introduce anything into the hydrocarbon stream which must later be removed.